Separating the stack vs the historical tasks.

- Adding a notion of session to the stack.  If the history is enabled,
  the stack will now be constrained to show either tasks that were last 
  active in a window of time, or a minimum number of tasks.
- Giving the history button the full space above the stack so that it 
  looks more balanced.

Change-Id: If7b647349e8bb164ae86fd6dba3f7e974257eb5b